The present invention pertains to a method of treating metal components. More particularly, the present invention pertains to a method of treating metal components by first building up a thickness of metal on a metal substrate using a Hyper Velocity Oxy-Fuel (HVOF) thermal spray process followed by a Hot Isostatic Pressing (HIP) heat treatment process.
Often during the manufacture of metal components a coating operation is performed to provide a coating material layer on the surface of a component substrate. The coating material layer is formed to build-up the metal component to desired finished dimensions and to provide the finished product with various surface attributes. For example, an oxide layer may be formed to provide a smooth, corrosion resistant surface. Also, a wear resistant coating, such as Carbide, Cobalt, or TiN is often formed on cutting tools to provide wear resistance.
Chemical Vapor Deposition is typically used to deposit a thin film wear resistant coating on a cutting tool substrate. For example, to increase the service life of a drill bit, chemical vapor deposition can be used to form a wear resistant coating of Cobalt on a high speed steel (HSS) cutting tool substrate. The bond between the substrate and coating occurs primarily through mechanical adhesion within a narrow bonding interface. During use, the coating at the cutting surface of the cutting tool is subjected to shearing forces resulting in flaking of the coating off the tool substrate. The failure is likely to occur at the marrow bonding interface.
FIG. 12(a) is a side view of a prior art tool bit coated with a wear resistant coating. In this case, the wear resistant coating may be applied by the Chemical Vapor Deposition method so that the entire tool bit substrate receives an even thin film of a relatively hard material, such as Carbide, Cobalt or TiN. Since the coating adheres to the tool bit substrate mostly via a mechanical bond located at a boundary interface, flaking and chipping off the coating off of the substrate is likely to occur during use, limiting the service life of the tool bit. FIG. 12(b) is a side view of a prior art tool bit having a fixed wear resistant cutting tip. In this case, a relatively hard metal cutting tip is fixed to the relatively soft tool bit substrate. The metal cutting tip, which is typically comprised of a Carbide or Cobalt alloy, is fixed to the tool bit substrate by brazing. During extended use the tool bit is likely to fail at the relatively brittle brazed interface between the metal cutting tip and the tool substrate, and again, the useful service life of the tool bit is limited.
Another coating method, known as Conventional Plasma Spray uses a super heated inert gas to generate a plasma. Powder feedstock is introduced and carried to the workpiece by the plasma stream. Conventional plasma spray coating methods deposit the coating material at relatively low velocity, resulting in voids being formed within the coating and in a coating density typically having a porosity of about 5.0%. Again, the bond between the substrate and the coating occurs primarily through mechanical adhesion at a bonding interface, and if the coating is subjected to sufficient shearing forces it will flake off of the workpiece substrate. Another coating method, known as the Hyper Velocity Oxyfuel (HVOF) plasma thermal spray process is used to produce coatings that are nearly absent of voids. In fact, coatings can be produced nearly 100% dense, with a porosity of less than 0.5%. In HVOF thermal spraying, a fuel gas and oxygen are used to create a combustion flame at 2500 to 3100.degree. C. The combustion takes place at a very high chamber pressure and a supersonic gas stream forces the coating material through a small-diameter barrel at very high particle velocities. The HVOF process results in extremely dense, well-bonded coatings. Typically, HVOF coatings can be formed nearly 100% dense, with at a porosity of &gt;0.5%. The high particle velocities obtained using the HVOF process results in relatively better bonding between the coating material and the substrate, as compared with other coating methods such as the Conventional Plasma spray method or the Chemical Vapor Deposition method. However, the HVOF process also forms a bond between the coating material and the substrate that occurs primarily through mechanical adhesion at a bonding interface.
Detonation Gun coating is another method that produces a relatively dense coating. Suspended powder is fed into a long tube along with oxygen and fuel gas. The mixture is ignited in a controlled explosion. High temperature and pressure is thus created to blast particles out of the end of the tube and toward the substrate to be coated.
Casting is a known method for forming metal components. Typically, a substrate blank is cast to near-finished dimensions. Various machining operations, such as cutting, sanding and polishing, are performed on the cast substrate blank to eventually obtain the metal component at desired finished dimensions. A casts metal component will typically have a number of imperfections caused by voids and contaminants in the cast surface structure. The imperfections may be removed by machining away the surface layer of the component, and/or by applying a surface coating.
All casting processes must deal with problems that the wrought processes do not encounter. Major among those are porosity and shrinkage that are minimized by elaborate gating techniques and other methods that increase cost and sometimes lower yield. However, the ability to produce a near net or net shape is the motivating factor. In some cases, it is more cost effective to intentionally cast the part, not using elaborate and costly gating techniques, and HIP treat the part to eliminate the sub-surface porosity. The surface of the part is then machined until the dense substrate is reached.
The manufacture of metal components often entails costly operations to produce products with the desired surface texture, material properties and dimensional tolerances. For example, a known process for manufacturing a metal component requires, among other steps, making a casting of the metal component, treating the metal component using a Hot Isostatic Pressing (HIP) treatment process, and then machining the metal component to remove surface imperfections and obtain the desired dimensional tolerances.
HIP treatment is used in the densification of cast metal components and as a diffusion bonding technique for consolidating powder metals. In the HIP treatment process, a part to be treated is raised to a high temperature and isostatic pressure. Typically, the part is heated to 0.6-0.8 times the melting point of the material comprising the part, and subjected to pressures on the order of 0.2 to 0.5 times the yield strength of the material. Pressurization is achieved by pumping an inert gas, such as Argon, into a pressure vessel. Within the pressure vessel is a high temperature furnace, which heats the gas to the desired temperature. The temperature and pressure are held for a set length of time, and then the gas is cooled and vented.
The HIP treatment process is used to produce near-net shaped components, reducing or eliminating the need for subsequent machining operations. Further, by precise control of the temperature, pressure and time of a HIP treatment schedule a particular microstructure for the treated part can be obtained.
Metal alloy components, such as gas turbine parts such as blades and vanes, are often damaged during use. During operation, gas turbine parts are subjected to considerable degradation from high pressure and centrifugal force in a hot corrosive atmosphere. The gas turbine parts also sustain considerable damage due to impacts from foreign particles. This degradation results in a limited service life for these parts. Since they are costly to produce, various repair methods are employed to refurbish damaged gas turbine blades and vanes.
Examples of methods employed to repair gas turbine blades and vanes include U.S. Pat. No. 4,291,448, issued to Cretella et al.; U.S. Pat. No. 4,028,787, issued to Cretella et al.; U.S. Pat. No. 4,866,828, issued to Fraser; and U.S. Pat. No. 4,837,389, issued to Shankar et al.
Cretella '448 discloses a process to restore turbine blade shrouds that have lost their original dimensions due to wear while in service. This reference discloses using the known process of TIG welding worn portions of a part with a weld wire of similar chemistry as the part substrate, followed by finish grinding. The part is then plasma sprayed with a material of similar chemistry to a net shape requiring little or no finishing. The part is then sintered in an argon atmosphere. The plasma spray process used in accordance with Cretella '448 results in a coating porosity of about 5.0%. Even after sintering the coating remains attached to the substrate and weld material only be a mechanical bond at an interface bonding layer making the finished piece prone to chipping and flaking.
Cretella '787 discloses a process for restoring turbine vanes that have lost their original dimensions due to wear while in service. Again, a conventional plasma spray process is used to build up worn areas of the vane before performing a sintering operation in a vacuum or hydrogen furnace. The porosity of the coating, and the interface bonding layer, results in a structure that is prone to chipping and flaking.
Fraser discloses a process to repair steam turbine blades or vanes that utilize some method of connecting them together (i.e. lacing wire). In accordance with the method disclosed by Fraser, the area of a part that has been distressed is removed and a new piece of like metal is welded to the part. The lacing holes of the part are plug welded. The part is then subjected to hot striking to return it to its original contour, and the lacing holes are re- drilled.
Shankar et al. disclose a process for repairing gas turbine blades that are distressed due to engine operation. A low-pressure plasma spray coating is applied to the vanes and the part is re-contoured by grinding. A coating of aluminum is then applied using a diffusion coating process. Again, the conventional low-pressure plasma spray process forms a mechanical bond at an interface boundary between the coating and the substrate, resulting in a structure that is prone to failure due to chipping and flaking.